1. Field of the Invention
The present invention relates to a loudspeaker apparatus and, more particularly, to a loudspeaker apparatus which achieves low-frequency sound reproduction using a small-size cabinet.
2. Description of the Background Art
Conventionally, audio apparatuses are becoming more digitalized, and players for reproducing music sources are becoming smaller and more portable. However, loudspeaker apparatus for eventually reproducing sounds require large cabinets so as to sufficiently reproduce sounds in a low frequency region included in music sources. Therefore, loudspeaker apparatuses carried in the small-size or portable players have small-volume cabinets, so that an acoustic stiffness exhibited by the cabinet is large, and therefore, it is difficult to achieve low-frequency sound reproduction to a sufficient extent.
Therefore, a loudspeaker apparatus has been disclosed in which a limit of low-frequency sound reproduction which is determined by the volume of a cabinet is improved (see, for example, Japanese Patent Laid-Open Publication No. 2000-308174). Hereinafter, the loudspeaker apparatus will be described with reference to FIG. 16. Note that FIG. 16 is a cross-sectional view of a structure of the loudspeaker apparatus.
In FIG. 16, the conventional loudspeaker apparatus roughly comprises a cabinet 101 and a loudspeaker unit 102. The loudspeaker unit 102 has a frame 103, an edge 104, a cone-shaped diaphragm 105, a dust cap 106, a voice coil bobbin 107, a damper 108, a voice coil 109, a magnet 110, a center pole 111, a magnetic plate 112, a movable magnet 113, and a fixed magnet 114.
In FIG. 16, the loudspeaker unit 102 is attached to an opening on a front surface of the cabinet 101. The magnet 110 is in the shape of a ring. A back surface of the magnet 110 (a surface of the magnet 110 closer to a back surface of the cabinet 101) is fixed to a front surface of the center pole 111. The magnetic plate 112 is in the shape of a ring. A back surface of the magnetic plate 112 is fixed to a front surface of the magnet 110. The voice coil 109 is wound around an outer circumferential surface of an end portion closer to the back surface of the voice coil bobbin 107. The voice coil 109 is placed in a magnetic gap formed between an outer circumferential surface of a convex of the center pole 111 and an inner circumferential surface of the magnetic plate 112. The frame 103 is fixed to a front surface of the magnetic plate 112. A sound hole 103h is formed in the frame 103. An outer circumference of the damper 108 is fixed to the frame 103. An inner circumference of the damper 108 is fixed to the voice coil bobbin 107. An inner circumferential surface of the cone-shaped diaphragm 105 is fixed to an end portion closer to the front surface of the voice coil bobbin 107. An inner circumference of the edge 104 is fixed to an outer circumference of the cone-shaped diaphragm 105. An outer circumference of the edge 104 is fixed to the frame 103. The dust cap 106 is fixed to a center portion closer to the front surface of the cone-shaped diaphragm 105. The movable magnet 113 is in the shape of a ring. An inner circumferential surface of the movable magnet 113 is fixed to an outer circumferential surface of the voice coil bobbin 107. The movable magnet 113 is provided between the cone-shaped diaphragm 105 and the damper 108, in the voice coil bobbin 107. The fixed magnet 114 is in the shape of a ring. An inner circumferential surface of the fixed magnet 114 faces an outer circumferential surface of the movable magnet 113, forming a gap. The movable magnet 113 and the fixed magnet 114 are magnetized to the same polarity in a thickness direction (vibration direction).
Next, an operation of the conventional loudspeaker apparatus will be described. When an electrical signal is applied to the voice coil 109, driving force is generated. The cone-shaped diaphragm 105 fixed to the voice coil bobbin 107 is vibrated by the driving force. Sound is generated from the cone-shaped diaphragm 105. The above-described operation is an operation of a typical electrokinetic loudspeaker. Here, two stiffnesses act on the cone-shaped diaphragm 105. These stiffnesses act in a direction which reduces a displacement of the cone-shaped diaphragm 105. The first stiffness is restoring force caused by the edge 104 and the damper 108 which support the cone-shaped diaphragm 105 (hereinafter, the restoring force is referred to as a support system stiffness S0). The second stiffness is force caused by the air in the cabinet 101 which is expanded/compressed by a displacement of the cone-shaped diaphragm 105, returning to the original state, i.e., force which puts the displacement of the cone-shaped diaphragm 105 back to the original state (hereinafter, the force is referred to as an acoustic stiffness Sc). The acoustic stiffness Sc is represented by:
                    Sc        =                              ρ            ⁢                                                  ⁢                          c              2                        ⁢                          π              2                        ⁢                          a              4                                V                                    (        1        )            where ρ represents a density of the air in the cabinet 101, c represents a sonic speed, a represents an effective radius of the cone-shaped diaphragm 105, and V represents an internal volume of the cabinet 101. The displacement of the cone-shaped diaphragm 105 is suppressed by the above-described two stiffnesses. Particularly, in the case of a loudspeaker apparatus having a small cabinet internal volume V, the acoustic stiffness due to the air in the cabinet is large. Therefore, it is difficult for the loudspeaker apparatus having a small cabinet internal volume V to reproduce a low-frequency sound region.
However, the conventional loudspeaker apparatus of FIG. 16 has a mechanism for generating force which reduces the acoustic stiffness acting on the cone-shaped diaphragm 105, i.e., a negative stiffness (hereinafter referred to as a negative stiffness generating mechanism). The negative stiffness generating mechanism is composed of the movable magnet 113 fixed to the outer circumferential surface of the voice coil bobbin 107, and the fixed magnet 114 provided facing the movable magnet 113. Hereinafter, the negative stiffness generating mechanism will be described in detail.
The movable magnet 113, when it is standing still (in the absence of a signal), is supported by a support system including the edge 104, the damper 108, and the like, and is located at a position where the movable magnet 113 and the fixed magnet 114 are magnetically balanced (hereinafter referred to as a balanced position). As described above, when an electrical signal is applied to the voice coil 109, the cone-shaped diaphragm 105 is vibrated by driving force generated in the voice coil 109. In this case, the movable magnet 113 is vibrated together with the voice coil bobbin 107 in an inner circumferential portion of the fixed magnet 114. Here, the movable magnet 113 and the fixed magnet 114 are magnetized to the same polarity in the vibration direction. Therefore, when the movable magnet 113 is displaced, a magnetic field in which the movable magnet 113 and the fixed magnet 114 repel each other is formed. Thereby, when the cone-shaped diaphragm 105 is displaced, force which allows the movable magnet 113 to escape from the balanced position, i.e., force which acts in a direction which increases the displacement, is generated in the movable magnet 113. Thus, the movable magnet 113 and the fixed magnet 114 constitute the negative stiffness generating mechanism.
As described above, the negative stiffness generating mechanism reduces the acoustic stiffness which acts on the vibration system of the loudspeaker unit 102. Thereby, the force which reduces the displacement of the cone-shaped diaphragm 105 is reduced, so that the volume V of expression (1) is equivalently increased. As a result, the loudspeaker apparatus of FIG. 16 can operate as if it carried a loudspeaker unit having a large cabinet.
Here, a specific method for expanding a reproduction band to a desired low-frequency region will be described. A low-frequency sound reproduction band of a loudspeaker apparatus is expanded by action of a negative stiffness. It is here considered that a loudspeaker apparatus with a cabinet having a volume V is used to obtain a low-frequency sound reproduction band equivalent to that of a cabinet having a volume N (N>1) times larger than V. In the case of the cabinet having the N-times volume, the acoustic stiffness value is represented by the following expression using expression (1).
                                          ρ            ⁢                                                  ⁢                          c              2                        ⁢                          π              2                        ⁢                          a              4                                NV                =                              1            N                    ⁢                      S            C                                              (        2        )            
Therefore, the magnitude of a negative stiffness required in the negative stiffness generating mechanism (a reduced amount of acoustic stiffness) is represented by the following expression.
                                          S            C                    -                                    1              N                        ⁢                          S              C                                      =                              (                                          N                -                1                            N                        )                    ⁢                      S            C                                              (        3        )            
Note that the negative stiffness indicated in expression (3) indicates a value of a linear component. As indicated in expression (3), when it is tried to obtain an effect of increasing the volume V by a factor of N (hereinafter referred to as a volume expansion effect), the smaller the volume V (or the larger the value N), the larger the negative stiffness which is required in the negative stiffness generating mechanism. Note that, by the negative stiffness generating mechanism, three stiffnesses act on the cone-shaped diaphragm 105. Two of the three stiffnesses are the support system stiffness S0 and the acoustic stiffness Sc. The third stiffness is a negative stiffness which is represented by expression (3) and is generated by the negative stiffness generating mechanism. A relationship between these three stiffnesses and the displacement of the movable magnet 113 is illustrated in FIG. 17. In FIG. 17, the horizontal axis indicates the displacement of the movable magnet 113. Regarding the displacement, a front surface direction of the cone-shaped diaphragm 105 is assumed to be a positive direction. The vertical axis indicates a magnitude of stiffness acting on the cone-shaped diaphragm 105. C in FIG. 17 indicates the resultant force of the acoustic stiffness Sc and the support system stiffness S0 acting on the cone-shaped diaphragm 105. The acoustic stiffness Sc is linear with respect to the displacement. The support system stiffness S0 is nonlinear since the edge 104 and the damper 108 are tense when the displacement is large. Therefore, C of FIG. 17 is nonlinear where the displacement is large. Next, D in FIG. 17 indicates a negative stiffness generated by the negative stiffness generating mechanism. An arrow in FIG. 17 indicates a portion of D where a nonlinear change occurs. A position of the nonlinear portion indicated with the arrow is determined, depending on a thickness of the fixed magnet 114. E in FIG. 17 indicates the resultant force of C and D. Therefore, a stiffness having a magnitude indicated by E of FIG. 17 acts on the cone-shaped diaphragm 105.
Here, in the conventional loudspeaker apparatus of FIG. 16, it is necessary to generate a larger negative stiffness so as to further expand a low-frequency sound reproduction band in a small-size loudspeaker apparatus. Here, to generate a larger negative stiffness, an outer diameter of the fixed magnet 114 of the negative stiffness generating mechanism may be increased, or the thickness thereof may be reduced. When the outer diameter of the fixed magnet 114 is increased, the cost of the fixed magnet 114 increases. Also, an increase in the outer diameter of the fixed magnet 114 leads to an increase in size of the loudspeaker unit 102. It is difficult for the large-size loudspeaker unit 102 to be incorporated into a small-size loudspeaker apparatus.
When the thickness of the fixed magnet 114 is reduced to generate larger negative stiffness, the position of the nonlinear portion indicated with the arrow in FIG. 17 is shifted toward a portion having a small displacement. Therefore, in E of FIG. 17, the linear range is narrowed. Here, if the amplitude of the cone-shaped diaphragm 105 exceeds the linear range, distortion of reproduced sound is increased, resulting in a reduction in reproduced sound quality. When the low-frequency sound reproduction band is expanded, the amplitude of the cone-shaped diaphragm 105 increases. Therefore, to suppress the distortion of reproduced sound, it is necessary to limit the amplitude of the cone-shaped diaphragm 105. In other words, when the thickness of the fixed magnet 114 is reduced, it is necessary to limit the amplitude, and therefore, the maximum sound pressure of reproduced sound decreases.
Another way to generate a larger negative stiffness, an inner diameter of the fixed magnet 114 may be increased and an outer diameter of the movable magnet 113 may also be increased, thereby enhancing the magnetic forces of both the magnets. However, the volumes of the magnets increase, leading to an increase in cost and an increase in weight of the vibration system. As a result, the efficiency of the loudspeaker apparatus decreases. For the above-described reasons, in the conventional loudspeaker apparatus of FIG. 16, there is a limitation on further expansion of the low-frequency sound reproduction band in a small-size loudspeaker apparatus, and it is difficult to achieve this.